1. Field of the Invention
This invention relates to a planetary-friction type speed change device arranged to have planetary rolling elements caused to revolve by the rotation of a sun roller and to give an output rotation at a speed reduced by the revolution of the planetary rolling elements.
2. Description of the Related Art
FIGS. 4 and 5 show by way of example the arrangement of the conventional planetary-friction type speed change device. FIG. 4 is a vertical section of the arrangement and FIG. 5 is a cross section of the same. These illustrations include a motor 1; a lower casing 2'; an upper casing 3'; a motor shaft 4; a sun roller 5 which is secured to the motor shaft 4; steel balls 6 which are planetary rolling elements; outer rings 7a' and 7b'; an elastic member 8 which is arranged to press the sun roller 5, the steel balls 6 and the outer rings 7a' and 7b' into contact with each other at a given amount of pressure for imposing a pre-load. A retainer 9 is arranged to keep the steel balls 6 at intervals of a given distance. The retainer 9 is secured to an output shaft 10.
The outer rings 7a' and 7b' are arranged coaxially with the sun roller 5. The steel balls 6 are respectively arranged between the outer circumferential surface of the sun roller 5 and the inner circumferential surface of each of the outer rings 7a' and 7b' and are approximately equally spaced by means of the retainer 9. The retainer 9 is fixedly connected to the output shaft 10 and is rotatably carried by an output-shaft bearing part 3a. The outer rings 7a' and 7b', the steel balls 6 and the sun roller 5 are kept in pressed contact under a given amount of pressure by the elastic member 8.
When the sun roller 5 rotates, the steel balls 6 revolve together with the retainer 9 while each of the steel balls 6 are rolling, because the outer rings 7a' and 7b', the steel balls 6, and the sun roller 5 are in contact with each other under the given amount of pressure. As a result, the output shaft 10 rotates at a reduced speed which is the same as the frequency of revolution of the steel balls 6.
It has been practiced to arrange the speed change device of this kind in the following manner: The diameter of the inner circumferential face of the lower casing 2' is set in such a way as to have the outer rings 7a' and 7b' in positions where they are coaxial with the sun roller 5. Then, the outer rings 7a' and 7b' are positioned and mounted by fitting them with pressure into the lower casing 2'. Such arrangement requires a high degree of precision for the inner diameter and outer diameter of the outer rings 7a' and 7b' and the diameter of the inner circumference of the lower casing 2'. These parts thus necessitate machining with a high degree of dimensional precision. This requirement causes an increase in cost. In addition to this shortcoming, another shortcoming of the conventional speed change device is that when eccentricity occurs between the sun roller 5 and the outer rings 7a' and 7b', an unsymmetrical contact of the steel balls 6, vibrations, and a decrease in efficiency result from the eccentricity. Further, the steel balls 6 roll on the inner circumferential faces of the outer rings 7a' and 7b' at a high speed while the speed change device is in operation. Since the outer rings 7a' and 7b' are directly in contact with the casings 2' and 3', the vibrations caused by the rolling movement of the steel balls 6 are transmitted to the outside of the speed change device through the outer rings 7a' and 7b' to cause vibrations and noise.